Semiconductor device and method for manufacturing the same

Abstract

An object is to improve field effect mobility of a thin film transistor using an oxide semiconductor. Another object is to suppress increase in off current even in a thin film transistor with improved field effect mobility. In a thin film transistor using an oxide semiconductor layer, by forming a semiconductor layer having higher electrical conductivity and a smaller thickness than the oxide semiconductor layer between the oxide semiconductor layer and a gate insulating layer, field effect mobility of the thin film transistor can be improved, and increase in off current can be suppressed.

What is claimed is: 
     
       1. A semiconductor device comprising:
 a gate electrode layer; 
 a gate insulating layer over the gate electrode layer; 
 a first oxide semiconductor layer over the gate insulating layer; 
 a second oxide semiconductor layer over the first oxide semiconductor layer; and 
 a pair of buffer regions having n-type conductivity over the second oxide semiconductor layer; 
 a source electrode layer over one of the pair of buffer regions and a drain electrode layer over the other of the pair of buffer regions, 
 wherein each of the second oxide semiconductor layer and the pair of buffer regions contains indium, gallium, and zinc, 
 wherein a thickness of the first oxide semiconductor layer is smaller than a thickness of the second oxide semiconductor layer, and 
 wherein an electrical conductivity of the first oxide semiconductor layer is higher than an electrical conductivity of the second oxide semiconductor layer. 
 
     
     
       2. The semiconductor device according to  claim 1 , wherein a carrier concentration of the second oxide semiconductor layer is lower than 1×10 17 /cm 3 . 
     
     
       3. The semiconductor device according to  claim 1 , wherein the first oxide semiconductor layer contains indium, gallium, and zinc. 
     
     
       4. The semiconductor device according to  claim 1 , wherein the electrical conductivity of the first oxide semiconductor layer is higher than 1.0×10 −3  S/cm 3 . 
     
     
       5. The semiconductor device according to  claim 1 , wherein the second oxide semiconductor layer has a region between the source and drain electrode layers, and wherein the region between the source and drain electrode layers has a thickness smaller than that of a region overlapping with the source and drain electrode layers. 
     
     
       6. The semiconductor device according to  claim 1 , wherein the semiconductor device is one selected from the group consisting of a poster, an advertisement, an electronic book, a television set, a digital photo frame, a game machine and a phone. 
     
     
       7. A semiconductor device comprising:
 a gate electrode layer; 
 a gate insulating layer over the gate electrode layer; 
 a first oxide semiconductor layer over the gate insulating layer; 
 a second oxide semiconductor layer over the first oxide semiconductor layer; 
 a pair of buffer regions having n-type conductivity over the second oxide semiconductor layer; and 
 a source electrode layer over one of the pair of buffer regions and a drain electrode layer over the other of the pair of buffer regions, 
 wherein each of the second oxide semiconductor layer and the pair of buffer regions contains indium, gallium, and zinc, 
 wherein a carrier concentration of the pair of buffer regions is higher than a carrier concentration of the second oxide semiconductor layer, 
 wherein a thickness of the first oxide semiconductor layer is smaller than a thickness of the second oxide semiconductor layer, 
 wherein an electrical conductivity of the first oxide semiconductor layer is higher than an electrical conductivity of the second oxide semiconductor layer, and 
 wherein an electrical conductivity of the pair of buffer regions is higher than the electrical conductivity of the first oxide semiconductor layer. 
 
     
     
       8. The semiconductor device according to  claim 7 , wherein the carrier concentration of the pair of buffer regions is 1×10 18 /cm 3  or higher. 
     
     
       9. The semiconductor device according to  claim 7 , wherein the carrier concentration of the second oxide semiconductor layer is lower than 1×10 17 /cm 3 . 
     
     
       10. The semiconductor device according to  claim 7 , wherein the first oxide semiconductor layer contains indium, gallium, and zinc. 
     
     
       11. The semiconductor device according to  claim 7 , wherein the electrical conductivity of the first oxide semiconductor layer is higher than 1.0×10 −3  S/cm 3 . 
     
     
       12. The semiconductor device according to  claim 7 , wherein the second oxide semiconductor layer has a region between the source and drain electrode layers, and wherein the region between the source and drain electrode layers has a thickness smaller than that of a region overlapping with the source and drain electrode layers. 
     
     
       13. The semiconductor device according to  claim 7 , wherein the semiconductor device is one selected from the group consisting of a poster, an advertisement, an electronic book, a television set, a digital photo frame, a game machine and a phone.